UCSB EEMB 2 - Lec #15 EEMB 2 SM17 1S (Speciation) (14 pages)

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Lec #15 EEMB 2 SM17 1S (Speciation)



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Speciation The Evolutionary Process Variation Natural Selection 1 Variation 2 Natural Selection 3 Genetic Divergence 4 Reproductive Isolation 5 Speciation Genetic Divergence Reproductive isolation Speciation Speciation Speciation 1 75 million species have been identified Changes in allele frequencies that are significant enough to mark the formation of daughter species from parent species Species How do we define a species Phylogeny line of descent Sp F Sp A Sp B Sp C Sp D Sp E Ex Poison dart frogs Dendrobates pumilio N S Leopard frogs Speciation Speciation 1 Biological Species Concept Ernst Mayer 1942 Species are groups of interbreeding natural populations that are reproductively isolated form other such groups the capacity to contribute to a shared gene pool is qualification for species membership Limitations this concept does not apply to asexually reproducing organisms bacteria some species of plants and fungi Western Meadowlark Eastern Meadowlark Ex Speciation Speciation 2 Morphological species concept based on morphological structural characteristics can be applied to sexual and asexual organisms Limitations Variation in morphology ontogenetic niche shifts sexual dimorphism makes this definition difficult Ex Bacteria Butterflies 3 Paleontological Species Concept Species are morphological discrete groups known only from the fossil record Limitations limited evidence not all organisms fossilize well Speciation Speciation 4 Ecological Species Concept Leigh Van Valen 1976 Species are separated by their ecological niche Limitations niche is hard to define and often flexible 5 Phylogenetic Species Concept Species are sets of organisms with a unique genetic history compares physical characteristics molecular sequences with those of other organisms limitations at what difference do we define a species It is not easy to define a species and there are multiple techniques to do so Method dependent on species under investigation Speciation Mechanisms of Speciation Speciation due to accumulation of genetic changes that promote significant differences between populations Depends on how gene flow between populations is interrupted Primary mechanisms of speciation 1 speciation due to abrupt events Ex changes in chromosomal 2 speciation due to adaptation to different environments RIMs Speciation Mechanisms of Speciation 1 Allopatric Speciation Gene flow is interrupted when a physical barrier between populations occurs that promotes the genetic changes necessary for speciation Physical barrier is effective at blocking gene flow between pops and depends on organisms ability to migrate Reproductive isolation arises in the genetically diverging populations Speciation is complete when individuals of the 2 populations can not interbreed mutation natural selection genetic drift promote genetic divergence Speciation Mechanisms of Speciation 1 Allopatric Speciation Physical barriers lead to reproductive isolation Small isolated populations are more likely than large populations to experience allopatric speciation adaptive radiation mutation natural selection genetic drift Adaptive radiation Upon introduction to various new environments ancestral species has evolved into wide array of descendent species that differ in morphology physiology and behavior Ex Galapagos Islands Ex Hawaiian Honey Creepers Speciation The Pace of Allopatric Speciation Timing of Speciation Measurable geographic distances separate the populations of most species Gene flow among populations is more an intermittent trickle than a steady stream Rapid Speciation punctuated equilibria Barriers arise rapidly and block immigration emigration between pops Ex Earthquake in 1800 s Mississippi River shift in course Gradual Speciation gradualism Barriers arise gradually block immigration emigration between pops determined by fossils and break up of contiguous habitats Ex Ice ages glaciation and interglaciation rise of mountain ranges Speciation Mechanisms of Speciation 2 Sympatric Speciation A species may form within the home range of an existing species in the absence of physical barriers polyploidy adaptation to local environments sexual selection Polyploidy During cell division an accident can result in extra sets of chromosomes results in immediate reproductive isolation relatively common in plants Autopolyploidy Non disjunction of chromosomes during gamete formation in the same species an individual that has more than 2 chromosome sets all derived from a single species Mechanisms of Speciation Polyploidy Autopolyploidy Ex Intraspecies breeding produces 2N diploid to 4N tetraploid plants via non disjunction of chromosomes 2N 4N cross produces 3N offspring that are sterile Repro Isolation 4N plants produce fertile offspring via self pollination or mating with other 4N individuals in one generation autopolyploidy can generate speciation Polyploidy Allopolyploidy common Ex Interspecies breeding produces a hybrid Usually sterile Hybrids 1 set of chromosomes from each species are sterile or infertile with each other but can not interbreed with either parental species sterile hybrids may propagate asexually and generate speciation or in subsequent generations a sterile hybrid can change into a fertile polyploid allopolyploidy can generate speciation New species Sterile Hybrid New species Speciation Sympatric Speciation Polyploidy Allopolyploidy Many Agricultural Crops Oats cotton potatoes tobacco wheat All are polyploids Wheat Triticum aestivum is an allohexaploid 6 sets of chromosome Polyploidy in animals rare event some frogs and fish Other mechanisms of sympatric speciation Genetic factors enable a subpopulation to exploit a resource not used by the parent population over time genetic differences accumulate leading to reproductive isolation and speciation Speciation Sympatric Speciation Adaptation and sexual selection Parapatric speciation adaptation to local environments and sexual selection promote speciation Neighboring populations become distinct species while maintaining contact along a common border Interbreeding individuals produce hybrid offspring in the border hybrid zone Ex African cichlid fishes in Rift Lakes Use the variety of species concepts to define when a species occurs Speciation Macroevolutionary Changes Speciation can result from very small genotypic and phenotypic differences microevolutionary process Species diverge and speciate repeatedly and these differences become more pronounced over time Speciation


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